"On behalf of" how can it be removed on child accounts??

Hello,

I have a ZCS 7.1.2 .
I have accounts with several child accounts in them. When I try to send a message from child accounts, the recipients recieve a email with "Primary Account<primaryaccount@domain.com> on behalf of Child Account <childaccount@domain-child.com>"

How can I remove that?
I want that the recipient to see only child account as the sender...